CMS Manufacturing Processes & Techniques 3 — Questions and Answers
Question 1: In injection molding, 'sink marks' on a plastic part are most commonly caused by:
- Excessive injection pressure
- Insufficient cooling time leading to localized shrinkage at thick sections (Correct answer)
- Too high a mold temperature causing flash
- Misaligned mold halves during clamping
Correct answer: Insufficient cooling time leading to localized shrinkage at thick sections
Sink marks form when thick sections cool and shrink inward before the outer skin solidifies, creating visible depressions on the part surface.
Question 2: Which honing process characteristic distinguishes it from grinding?
- Honing uses a fixed abrasive wheel rotating at high speed
- Honing uses abrasive stones with reciprocating and rotating motion at low speed to improve bore geometry (Correct answer)
- Honing is used exclusively for external cylindrical surfaces
- Honing removes more material per pass than grinding
Correct answer: Honing uses abrasive stones with reciprocating and rotating motion at low speed to improve bore geometry
Honing uses bonded abrasive stones that simultaneously rotate and reciprocate at relatively low speeds, primarily improving bore geometry, surface finish, and size accuracy.
Question 3: The Taylor tool life equation (VT^n = C) describes the relationship between cutting speed and tool life. What does a higher value of the exponent 'n' indicate?
- The tool material is more sensitive to cutting speed changes
- The tool material is less sensitive to cutting speed — tool life changes less with speed variation (Correct answer)
- The cutting fluid is more effective at higher speeds
- The workpiece material is harder and more abrasive
Correct answer: The tool material is less sensitive to cutting speed — tool life changes less with speed variation
A higher 'n' value (closer to 1) means tool life is less sensitive to changes in cutting speed, indicating a more thermally stable tool-workpiece combination.
Question 4: In sheet metal bending, 'springback' refers to:
- The material fracturing along the bend line during forming
- The elastic recovery of the bent sheet that reduces the final bend angle after the load is removed (Correct answer)
- Thinning of the sheet at the bend radius
- Wrinkling of the sheet on the compression side of the bend
Correct answer: The elastic recovery of the bent sheet that reduces the final bend angle after the load is removed
Springback is the elastic recovery that occurs when the bending force is released, causing the part to partially return toward its original shape and reducing the achieved bend angle.
Question 5: Which non-destructive testing (NDT) method is most effective for detecting subsurface cracks in non-ferromagnetic conductive materials like aluminum?
- Magnetic particle inspection (MPI)
- Eddy current testing (Correct answer)
- Dye penetrant inspection (DPI)
- Visual inspection
Correct answer: Eddy current testing
Eddy current testing uses electromagnetic induction to detect subsurface discontinuities in conductive materials, including non-ferrous metals like aluminum.
Question 6: In forging operations, 'flash' is the thin fin of excess material that forms at the parting line. Its primary function during closed-die forging is to:
- Indicate that the die has completely filled
- Create back-pressure that forces metal to fill all die cavities completely (Correct answer)
- Reduce forging force by providing a pressure relief path
- Improve surface finish of the forged part
Correct answer: Create back-pressure that forces metal to fill all die cavities completely
Flash creates resistance in the flash gutter, generating back-pressure that forces the workpiece material to flow into and completely fill all details of the die cavity.
Question 7: What is the primary difference between plasma arc cutting and oxyfuel cutting?
- Oxyfuel cutting is faster and more precise for thick steel
- Plasma arc cutting uses ionized gas at high temperatures and can cut any electrically conductive material, including stainless steel and aluminum (Correct answer)
- Plasma arc cutting requires preheating the workpiece before cutting
- Oxyfuel cutting produces a narrower kerf and less heat-affected zone
Correct answer: Plasma arc cutting uses ionized gas at high temperatures and can cut any electrically conductive material, including stainless steel and aluminum
Plasma arc cutting melts and expels material using a high-temperature ionized gas jet, making it effective on any conductive metal, while oxyfuel relies on oxidation and is limited to carbon steels.
CMS Manufacturing Processes & Techniques 3
In injection molding, 'sink marks' on a plastic part are most commonly caused by: